A wavelength up-conversion glass ceramic and a process for the production thereof

Abstract: A high efficiency wavelength up-conversion transparent glass ceramic composition containing rare earth ions, which can be applied for example to short wavelength solid lasers, full color displays and infrared light detecting sensors, has the following chemical composition, in which fluoride fine crystals containing rare earth ions are preferentially precipitated: SiO₂ 10 - 60 mol % AlO 1.5 0 - 40 mol % GaO 1.5 0 - 40 mol % PbF₂ 5 - 60 mol % CdF₂ 0 - 60 mol % GeO₂ 0 - 30 mol % TiO₂ 0 - 10 mol % ZrO₂ 0 - 10 mol % ReF₃ or ReO 1.5 (Re = Er, Tm, Ho, Yb, Pr, or other rare earth element) 0.05 - 30 mol %

Description: This invention relates to a high efficiency wavelength up-conversion transparent glass ceramic composition containing rare earth ions, which can be applied for example to short wavelength solid lasers, full color displays and infrared light detecting sensors, and a process for the production thereof. Of late, wavelength up-conversion materials utilizing the electronic transition between a plurality of energy levels of rare earth ions have been studied with keen interest because they can be applied in various fields such as blue or green solid lasers, full color displays and infrared light detecting sensors. As transparent wavelengths up-conversion materials with a relatively high conversion efficiency, fluoride single crystals and glasses have hitherto been known. However, it Is Impossible to produce a fluoride single crystal having an optical homogeneity and practical large size on a commercial scale and in an economical manner, since although the fluoride single crystal is excellent in conversion efficiency, mechanical strength and chemical stability, it is difficult to grow a high quality one. On the other hand, since fluoride glass In the form of a fiber Is capable of effectively sealing an excitation light in a fiber core, a visible light fiber laser having the highest infrared conversion efficiency has been obtained at the present time. However, fluoride glass is inferior to the fluoride single crystal in thermal, mechanical and chemical stability and has problems in durability and reliability, for example. Thus, fluoride glass tends to deteriorate by erosion with water and when subjected to irradiation bya laser beam of large power, it is very prone to be damaged. For the preparation of a fluoride glass fiber, furthermore, precise control of the preparation conditions such as temperature and atmosphere, is essential thus resulting in increase of production cost. On the other hand, as a glass having a very high stability is known a composition, typical of optical glasses, comprising, as a predominant component, oxides. Ordinary optical glasses containing the so-called glass-forming materials having very high chemical bonding strength, such as SiO 2 , GeO 2 , AlO 1.5 , BO 1.5 , PO 2.6 , exhibit high viscosity and are much more superior in moldability, water resistance, and mechanical strength, than fluoride glasses containing ionic bonding compounds. In oxide glasses, however, the emission efficiency from a number of rare earth ion levels is significantly lower than that of fluorides, and oxide glasses are not suitable for use as applying devices utilizing emission of rare earth ions, for example, laser materials. As wavelength up-conversion materials of the prior art, glass consisting of high quality fluoride meets with higher production cost and oxide glass with high stability meets with lower emission efficiency. It is an object of the present invention to provide a high efficiency wave-length up-conversion transparent glass ceramic composition containing rare earth ions. It is another object of the present invention to provide a transparent material having excellent wavelength up-conversion characteristics as well as high stability, whereby the problems of the prior art can be solved. It is a further object of the present invention to provide a process for the production of a high efficiency wave-length up-conversion transparent glass ceramic composition containing rare earth ions. These objects can be attained by a transparent glass ceramic composition having the following chemical composition, in which fluoride fine crystals having sizes smaller than the wavelength of light containing rare earth ions are preferentially precipitated: SiO 2 10 - 60 mol % (preferably 20 - 45 mol %) AlO 1.5 0 - 40 mol % (preferably 10 - 30 mol %) GaO 1.5 0 - 40 mol % (preferably 10 - 30 mol %) PbF 2 5 - 60 mol % (preferably 20 - 50 mol %) CdF 2 0 - 60 mol % (preferably 10 - 40 mol %) GeO 2 0 - 30 mol % (preferably 0 - 20 mol %) TiO 2 0 - 10 mol % (preferably 0 - 6 mol %) ZrO 2 0 - 10 mol % (preferably 0 - 6 mol %) ReF 3 or ReO 1.5 (Re = Er, Tm, Ho, Yb, Pr, or other rare earth element) 0.05 - 30 mol % (preferably 0.5 - 20 mol %) and by a process for the production of a transparent glass ceramic composition, comprising preparing an oxide-fluoride glass containing rare earth ions by a conventional melting method, subjecting the glass to a heat treatment at a temperature higher than the glass transition temperature and thereby precipitating preferentially fluoride fine crystals having sizes smaller than the wavelength of light containing a large amount of rare earth ions. The accompanying drawings illustrate the principle and merits of the present invention in detail. Fig. 1 is an example of emission spectra when Example 1 of the infrared-to-visible up-conversion material of the present invention is excited by an infrared light of wavelength 980 nm [(a) before heat treatment, (b) after heat treatment and (c) Er-Yb added fluoride glass according to Comparative Example 1]. Fig. 2 shows scattering curves of the material of Example 1 (a) before heat treatment and (b) after heat treatment. Fig. 3 shows transmission spectra of the material of Example 1 (a) before heat treatment and (b) after heat treatment. Fig. 4 shows the excitation power dependence of the emission intensity of the glass ceramic and fluoride glass according to Example 1 and Comparative Example 1. Fig. 5 shows visible light emission spectra when (a) the material of Example 2 and (b) Tm 3+ - and Yb 3+ -added fluoride glass are excited by an infrared light of wavelength 980 nm. The inventors have made various efforts to develop a high efficiency wavelength up-conversion transparent glass ceramic composition containing rare earth ions to be applied to short wavelength solid lasers, full color displays and infrared light detecting sensors and have discovered the transparent glass ceramic composition having the foregoing chemical composition, in which fluoride fine crystals containing rare earth ions are preferentially precipitated. In the foregoing chemical composition according to the present invention, SiO 2 and AlO 1.5 or GaO 1.5 are important as glass raw materials for forming the network structure of the transparent glass matrix and with increase of the content of SiO 2 , the glass tends to be formed, but desired fluoride fine crystals are difficult to precipitate. In particular, when SiO 2 is at least 60 mol % or AlO 1.5 or GaO 1.5 is at least 40 mol %, the fine crystals cannot be precipitated and the emission efficiency is decreased. PbF 2 and CdF 2 not only promote formation of the glass, but also play an essential role in precipitating the fine crystals. When more than 60 mol % of PbF 2 and CdF 2 are added, a glass is not formed. GeO 2 is a component playing the same role as SiO 2 and even when a part of SiO 2 is replaced by GeO 2 , the formation of the glass Is not affected. TiO 2 or ZrO 2 is effective for the precipitation of the fine crystals even If added In a small amount and when adding at least 10 mol %, this component is not dissolved. Rare earth elements (Re +3 ) are emission components, of which Yb +3 as a sensitizer is preferably used in a larger amount, e.g. about 20 mol % and other Re 3+ elements are preferably used in an amount of up to 5 mol % so as to prevent concentration quenching. In the present invention there is provided a process for the production of a transparent glass ceramic composition, comprising preparing an oxide-fluoride glass containing rare earth ions by a conventional melting method, subjecting the glass to a heat treatment at a temperature higher than the glass transition temperature and thereby precipitating preferentially fluoride fine crystals having sizes smaller than the wavelength of light containing a large amount of rare earth ions. Specifically, glass-forming materials, for example, powders of SiO 2 , AlO 1.5 and fluorides such as PbF 2 , CdF 2 , ReF 3 (rare earth element fluorides) are mixed, charged to a platinum crucible, solubilized in the air at a temperature of about 1000 °C, cast in a mold of carbon and charged to an annealing furnace to remove the strain. Up to this step, substantially the same process is carried out as in ordinary processes for the production of oxide glasses, thus obtaining transparent glasses with various shapes. When the thus resulting transparent glass is further heat treated for several hours at a temperature higher than the glass transition temperature, at which no devitrification occurs, the emission efficiency of the rare earth elements can at once be increased without diminishing the transparency. In such a very simple method, a transparent body with a high rare earth emission efficiency can be obtained, because a large amount of fluoride fine crystals containing rare earth ions are precipitated in the glass by the heat treatment. Since a large amount of the fine crystal grains precipitated in the resulting transparent glass ceramic have sizes smaller than the wavelengths of light, however, scattering of visible light is so little that it can be ignored and the glass apparently has the same complete transparency before and after the heat treatment, although differing in structure. Concerning the components, moreover, GeO 2 or BO 1.5 can be used as a raw material or raw component instead of SiO 2 ; GaO 1.5 or TiO 2 instead of AlO 1.5 and rare earth oxides ReO 1.5 instead of the rare earth fluorides ReF 3 . The important point is to stably incorporate fluorides into an oxide glass, subjecting the mixture to a heat treatment and thereby precipitating fluoride fine crystals containing rare earth ions. Heavy metal fluorides such as PbF 2 , CdF 2 , T1F, etc. are preferable as the component, since they metastably form ionic bonds with fluoride ion in the oxide glass network and function to prevent the rare earth ions from direct bonding with Si-O. The atmosphere used for melting the glass is not limited, but can be air. As crucibles, there can be used crucibles of platinum, alumina, or silica. The above described production process will specifically be illustrated in the following. Powders of oxides and fluorides as raw materials are mixed, charged to a platinum crucible provided with a cover and uniformly melted at about 1000 °C. When the content of AlO 1.5 , TiO 2 or ZrO 2 each having a high melting point is larger, it is preferable to adjust the melting temperature somewhat higher (less than 1200 °C). If the temperature is higher than 1200 °C, however, a loss of the fluoride due to evaporation is so increased that the melt should be prevented from reaching such a high temperature. The melting time, depending upon the amount of one batch, is generally 1 to 2 hours in the case of about 100 g, and with the increase of the amount thereof, the melting time is preferably lengthened to homogenize the mixture. However, the maintenance of the melt for a longer period of time should be avoided since the loss of fluorides due to evaporation is significant. The glass melt is cast in a mold, solidified, annealed near the glass transition temperature (Tg), cooled therefrom to a temperature of about 50 °C lower than Tg, further heated and heat-treated at a temperature of about 100 °C higher than Tg and thus fine crystals are precipitated. Generally, Tg is different depending on the composition of the glass and accordingly, the heat treatment temperature is different according to the glass composition. As long as denitrification does not occur, it is preferable to effect the heat treatment at a higher temperature from the standpoint of efficiency and at a higher temperature the heat treatment time is shorter, but in general, the heat treatment should preferably be carried out for at least 10 hours. Examples The present invention will now be illustrated in greater detail by the following examples, but the present invention and merits thereof are not intended to be limited by the materials, compositions and production procedures described in these examples. Of course, these examples are only given to exemplify the present invention and can variously be changed or modified without departure from the scope of the present invention. Example 1 Powders of SiO 2 , Al(OH) 3 , PbF 2 , CdF 2 , YbF, and ErF 3 were weighed and mixed to give a composition of 30 % SiO 2 , 15 % AlO 1.5 , 24 % PbF 2 , 20% CdF 2 , 10 % YbF 3 and 1 % ErF 3 (mol %). The resulting mixture was charged to a platinum crucible and melted for about 1 hour in air at a temperature of 1050 °C. The unifomly melted mixture was cast in a mold of carbon and annealed at 400 °C. The thus obtained transparent glass was further heat treated at a temperature of 470 °C for 7 hours and slowed to stand and cool at room temperature. Fig. 1 shows emission spectra when (a) the glass free from the heat treatment and (b) the same glass but heat-treated at 470 °C are irradiated by a semiconductor laser with a wavelength of 980 nm. The emissions at a wave-length band of 550 nm and 660 nm are respectively green and red emission by Er 3+ and in particular, the emission of 550 nm is expected to be applied to short wavelength lasers and the like. It is apparent from Fig. 1 that the emission intensity in the visible range is improved by at least about 100 times by the heat treatment at 470 °C. Fig. 2 shows X-ray scattering curves of the samples before and after the heat treatment in this Example. It is shown therein that the sample before the heat treatment (a) gives a broad curve characteristic of a glass, while the heat-treated sample (b) gives sharp peaks due to the presence of crystals. That is, this shows that a large amount of crystalline material is precipitated in the glass by the heat treatment. These peaks are due to fine crystals of fluoride solid solutions of PbF 2 , CdF 2 , YbF 3 , ErF 3 , etc. and it is calculated from the half value width thereof that the size of the fine crystals is very small. i.e. of the order of 20 nm. Fig. 3 shows transmission spectra of the glass sample before and after the heat treatment, in which the dotted line shows the result after the heat treatment and the solid line shows that before the heat treatment. It is apparent from this figure that the transmittance is hardly changed before and after the heat treatment. Generally, when foreign matter such as crystals are precipiated in a glass, the transmission is significantly lowered by scattering of light, but if the precipitated crystals are very small, e.g. smaller than the wavelength of light and the difference in refractive index between the fine crystals and matrix glass is small, the loss of transmission due to scattering of light can be so suppressed that it can be ignored. The result of Fig. 3 is the latter case. Fig. 4 shows the excitation power dependence of the visible light emission measured using the material according to Example 1 and a semiconductor laser of 980 nm as an excitation light source. All the emissions of 550 nm and 660 nm are due to the energy transition among a plurality of energy levels of rare earth ions (In this case, 4 I 11/2 , 4 S 3/2 and 4 F 3/2 levels of Er 3+ and 2 F 5/2 level of Yb 3+ ), which shows the square dependence of the excitation power characteristic of the wavelength up-conversion. In Fig. 4, the symbols have the following meanings: □ : I 410 , transparent glass ceramics before heat treatment ○ : I 650 , transparent glass ceramics before heat treatment Δ : I 660 , transparent glass ceramics before heat treatment ▪ : I 410 , transparent glass ceramics after heat treatment  : I 550 , transparent glass ceramics after heat treatment ▴ : I 660 , transparent glass ceramics after heat treatment smaller ▪ : I 450 , fluoride glass of the prior art smaller  : I 550 , fluoride glass of the prior art smaller ▴ : I 650 , fluoride glass of the prior art "I x " : I = Intensity, x = Emission Wavelength "Pin" : P = Power (Excitation Light Power), in = input Comparative Example 1 For comparison, a typical fluoride glass hitherto developed (35 % AlF 3 , 14 % YbF 3 , 1 % ErF 3 , 20 % PbF 2. 5 % MgF 2 , 15 % CaF 2, 10 % BaF 2, mol %) was prepared using a platinum crucible in a nitrogen atmosphere. It has been confirmed that this glass is not stable because it contains no compound with strong bonding property, but has a high infrared-to-visible conversion efficiency with the same level as the fluoride single crystal. In Fig. 1 (c) and Fig. 4 are respectively shown the emission spectra of the glass when this is irradiated by a semiconductor laser of 980 nm and the excitation power dependence of the emission intensity. The green emission intensity of the fluoride glass was about 1/2 as large as that of the transparent glass ceramic of Example 1 and the power dependences of both the glasses were substantially same. Thus, it was confirmed that the material of the present Invention exhibited a higher efficiency over from a lower excitation power to a higher excitation light power than the fluoride glass. Example 2 and Comparative Example 2 A transparent glass ceramic having a composition of 30 % SiO 2 , 15 % AlO 1.5, 45 % PbF 2 , 10 % YbF 3 and 0.1 % TmF 3 (mol %) was prepared In an analogous manner to Example 1. When this sample was Irradiated by a laser of 980 nm, blue emission, one of the three primary colors, was strongly observed. This blue emission spectrum is shown in Fig. 5. For comparison, an emission spectrum of a Yb 3+ - and Tm 3+ -co-doped fluoride glass (35 % AlF 3 , 15 % YbF 3 , 20 % PbF 2 , 5 % MgF 2 , 15 % CaF 2 , 10 % BaF 2 , 0.1 % TmF 3 , Comparative Example 2) by 980 nm excitation is also shown in Fig. 5. All the emissions in the visible range are due to Tm 3+ . As to the emission intensity of 360 nm, the glass ceramic is stronger than the fluoride glass, but both the glasses show substantially the same blue emission of 480 nm. In Fig.5, (a) shows an emission spectrum of the glass ceramics of Example 2 and (b) shows that of Comparative Example 2. Examples 3-6 Samples of Examples 3-6 were prepared in an analogous manner to Example 1 and subjected to measurement of the infrared-to-visible wavelength conversion performance, e.g. the excitation power dependence S x of the visible light emission intensity when excited by a 980 nm laser, thus obtaining results as shown in Table 1 with the data of Examples 1 and 2. According to the present invention, there can readily be obtained a transparent glass ceramic with a high infrared-to-visible conversion efficiency and high chemical, mechanical and thermal stability, which can be applied to short wavelength solid lasers, full color displays, infrared light detecting sensors, etc. Examples 1 3 4 2 5 6 Comparative Examples 1 2 SiO 2 30 35 30 30 35 30 AlO 1. 5 15 15 15 15 15 15 PbF 2 24 39 44 45 40 40 CdF 2 20 YbF 3 10 10 10 10 10 10 ErF 3 1 1 1 TmF 3 0.1 0.1 HoF 3 0.1 S 410 2.60 2.62 2.60 2.74 S 545 1.68 1.82 1.85 1.87 S 655 2.14 2.13 1.90 2.12 S 363 2.81 2.96 3.07 S 451 2.78 2.93 2.95 S 477 1.82 2.13 2.03 S 650 1.82 2.14 2.03 Emission Color (A) (A) (A) (B) (3) (C) (C) (B) Emission Intensity (D) (D) (D) (D) (D) (D) (D) (D) Note: (A): yellow green; (B): blue; (C): green (D): strong

Claims: A transparent glass ceramic composition having the Tollowing chemical composition, in which fine, rare earth containing fluoride crystals having sizes smaller than the wavelength of light are precipitated in a glass matrix: SiO 2 10 - 60 mol % AlO 1.5 0 - 40 mol % GaO 1.5 0 - 40 mol % PbF 2 5 - 60 mol % CdF 2 0 - 60 mol % GeO 2 0 - 30 mol % TiO 2 0 - 10 mol % ZrO 2 0 - 10 mol % ReF 3 or Re0 1.5 (Re = Er, Tm, Ho, Yb, Pr, or other rare earth element) 0.05 - 30 mol % A process for the production of a transparent glass ceramic composition. comprising preparing an oxide-fluoride glass containing rare earth ions by a conventional melting method, subjecting the glass to a heat treatment at a temperature higher than the glass transition temperature and thereby precipitating preferentially fluoride fine crystals having sizes smaller than the wavelength of light containing a large amount of rare earth ions. A process as claimed in Claim 2, wherein glass forming materials are mixed, charged to a platinum crucible, solubilized in the air at a temperature of about 1000 °C, cast in a mold of carbon and charged to an annealing furnace to remove the strain and the thus resulting transparent glass is further heat treated for several hours at a temperature higher than the glass transition temperature, at which no devitrification occurs, whereby the emission efficiency of the rare earth elements can be increased without diminishing the transparency. Use of a ceramic composition as claimed in Claim 1 in the manufacture of a short wavelength solid laser, a full color display or an infrared light detecting sensor.

Inventors: OHWAKI JUNICHI, WANG YUHU, OHWAKI, JUNICHI, WANG, YUHU, OHWAKI, JUNICHI, C/O NIPPON TELEGRAPH AND, WANG, YUHU, SUMITA OPTICAL GLASS, INC.
Applicants: NIPPON TELEGRAPH & TELEPHONE, SUMITA OPTICAL GLASS, NIPPON TELEGRAPH AND TELEPHONE CORPORATION, SUMITA OPTICAL GLASS, INC.
Application year: 1994
Main classification: C03C3
IPC codes: H01S3, C03C10, C03C3, C03C4